A receiver transmits an acknowledgement (ACK) signal to a transmitter when the receiver succeeds in demodulating received data, and transmits a negative acknowledgement (NAK) signal to the transmitter when the receiver falls to demodulate the received data. Each of the ACK/NAK signals is expressed as one bit per codeword. The ACK/NAK signals should be enabled to be simultaneously transmitted by a plurality of users (terminals) using given time and frequency resources through multiplexing.
Such multiplexing techniques are classified into frequency division multiplexing (FDM) and code division multiplexing (CDM). FDM is a form of multiplexing where a plurality of different terminals use different time/frequency resources, whereas CDM is a form of multiplexing where a plurality of different terminals use the same time/frequency resources but transmit results obtained by multiplying signals by specific orthogonal codes so that a receiver can identify the plurality of different terminals.
In an uplink, a Zadoff-Chu sequence having an ideal peak to average power ratio (PAPR) is often used. Such a Zadoff-Chu sequence can achieve orthogonality between terminals through a cyclic delay, instead of multiplying a signal by a specific code in a frequency domain.
An uplink ACK/NAK signal is required for a terminal to inform a base station of a successful or unsuccessful (ACK or NAK) receipt of downlink data, and requires one bit per codeword which is used to transmit the downlink data.
FIG. 1 illustrates time/frequency resources used by a terminal to perform uplink ACK/NAK signaling in a 3rd generation partnership projection long term evolution (3GPP LTE) system. Referring to FIG. 1, resources used by one control channel are grouped into two separate resource blocks. Each of the two resource blocks includes N subcarriers along a frequency axis, and 7 orthogonal frequency division multiplexing (OFDM) symbols, which corresponds to one slot, along a time axis. One slot has a time duration of 0.5 ms.
In FIG. 1, a plurality of terminals may commonly use one control channel. That is, a control channel A or a control channel B may be shared by the plurality of terminals.
In this case, in order to identify the plurality of terminals using the same control channel, a specific code sequence is allocated to each of the plurality of terminals. That is, each of the plurality of terminals generates and transmits a signal spread along a frequency axis and a time axis by using its allocated specific code.
FIG. 2 illustrates a code sequence and a symbol transmitted to each of N subcarriers in an ACK/NAK channel occupying a resource block that includes the N subcarriers along a frequency axis and 7 OFDM symbols along a time axis. In FIG. 2, the resource block corresponding to one slot described with reference to FIG. 1 occupies N subcarriers on a frequency axis and includes 7 symbol blocks BL #0 through #6 on a time axis.
When CDM is used to identify signals of a plurality of terminals, a sequence and a symbol may be mapped to each time/frequency resource as illustrated in FIG. 2. In order to identify the signals of the plurality of terminals, a sequence is applied to each of the frequency axis and the time axis. In FIG. 2, a reference signal is used for channel estimation, and a pre-determined signal between a terminal and a base station is transmitted.
The base station estimates a channel by the reference signal, and uses a result of the channel estimation so as to demodulate an ACK/NAK symbol transmitted by a control signal. Each time/frequency resource carrys out a signal multiplied by two or three symbols.
That is, a time/frequency resource on which the reference signal is carried, is obtained by multiplying a frequency axis sequence symbol Cqm(k) by a time axis sequence symbol Ri (i=0, 1, 2). A time/frequency resource on which the control signal is carried, is obtained by multiplying a frequency axis sequence symbol Cqm(k), a time axis sequence symbol Ci (i=0, 1, 2, 3), and an ACK/NAK symbol Q.
In FIG. 2, the frequency axis sequence symbol Cqm(k) indicates a Zadoff-Chu sequence where NZC is the length of the Zadoff-Chu sequence applied to a kth subcarrier along the frequency axis, m is a primary index, and q is a cyclic delay index, and is provided by Equation 1.
                                                        C              q              m                        ⁡                          (              k              )                                =                      exp            ⁡                          [                              i                ⁢                                                      2                    ⁢                    π                                                        N                    ZC                                                  ⁢                                  m                  ⁡                                      (                                                                                            (                                                      k                            -                            q                                                    )                                                ⁢                                                  (                                                      k                            -                            q                            +                            1                                                    )                                                                    2                                        )                                                              ]                                      ,                                  ⁢                  k          =          0                ,        1        ,        2        ,        …        ⁢                                  ,                  N          -          1                                    [                  Equation          ⁢                                          ⁢          1                ]            
One sequence is applied to each of the reference signal and the control signal along the time axis. That is, a sequence applied to the control signal in FIG. 2 is expressed as C0, C1, C2, and C3. A sequence applied to the reference signal is expressed as R0, R1, and R2.
Currently, 3GPP LTE considers a configuration in which three reference signals per slot are used for an uplink ACK/NAK channel.
Also, in order to identify a plurality of terminals, a Zadoff-Chu sequence is used along a frequency axis, and a discrete Fourier transformation (DFT) vector, a Walsh-Hadamard sequence, or a Zadoff-Chu sequence may be used along a time axis.